Transcoder: Fix error with short clips

- Fixes a bug in the sample reader where an error was incorrectly
reported if one track reached EOS right before switching to
sequential access.
- Adds more sample reader tests for different combinations of
sample access patterns and access modes.

Bug: 153453392
Fixes: 173643110
Test: Unit test (MediaSampleReaderNDKTests)
Change-Id: I3b683c5d8eb18a5b57d419ce113e08b40363ba9e